The invention relates to cryogenically-cooled high voltage, high power electronic switching circuitry.
Efforts in utilizing Metal Oxide Semiconductor Field Effect Transistor (MOSFET) switching devices in cryogenically-cooled high power conversion circuits are ongoing. It has been appreciated that these particular devices, when cryogenically cooled, have unique advantages not found in other conventional high power semiconductor switching devices, including IGBTs, MCTs, GTOs and bipolar transistors (see U.S. Pat. No. 5,126,830, Mueller et al. and U.S. Pat. No. 5,347,168, Russo). For example, power MOSFET's, when operated at a temperature of 77 K, exhibit a reduction of the on-resistance of the MOSFET's typically by a factor of 15 or more, and often by as much as a factor of 30, resulting in a significant decrease of the conduction losses within the devices.
One particularly advantageous use of cryogenically cooled electronics is in high power, switching power supplies. To understand the advantages in more detail, however, a brief discussion of the limitations of such power supplies operating at room temperature needs to be considered. Switch mode amplifiers, regulated power supplies, and frequency converters became a reality with the introduction of high speed power silicon devices. An important advantage of these switch mode applications is that, at least for ideal devices, the only losses involved are the saturation losses of the power devices in the forward direction known as the conduction losses. These conduction losses are very low compared to the losses sustained in linear regulation or amplification devices, thereby resulting in a considerable reduction in the physical size of regulated power supplies and an increase of operating efficiency. There are, however, other losses associated with switch mode converters including the commutation losses associated with the switching device, switching losses, and parasitic discharge losses. The commutation, parasitic discharge, and switching losses are all proportional to the switching frequency. Moreover, capacitive discharge losses may dominate, particularly at high operating voltages, because they are proportional to the square of the applied voltages.
The relationship between switching loss and frequency is important since the typical method used to obtain size reductions in switch mode power supplies is to operate at high switching frequencies which permits replacing conventional power frequency components with significantly smaller filter and active components. These smaller components operate at 300 to 30,000 times the frequency of the older power supplies. At these increased frequencies, the switching losses in the power devices often dominate the overall loss of the power supply.
A switch mode converter may include a commutating diode whose forward voltage drop generates additional losses, referred to as commutating diode losses. The commutating diodes provide a path for the inductive component of load current to flow when the load current is interrupted by turn-off of a switching device. With certain unidirectional high power devices (e.g., IGBTs, GTOs, thyristor diodes) the commutating diodes are added externally.
Soft-Switching techniques which generate so-called Zero-Voltage Switching (ZVS) or Zero Current Switching (ZCS) conditions have been used to reduce the switching losses of switching devices. The switching losses are reduced because the switching of the active device occurs at a point in time when, as the names imply, either the voltage or the current is zero, thus providing a zero power dissipation condition. These techniques are, in general, "waveshaping" circuits having a suitable inductor which resonates or "quasiresonates" with the output capacitances of the active devices in a manner that the ZVS or ZCS conditions are obtained. In the case of capacitive discharge losses, the resonant techniques assure that the capacitances are discharged, not through the active switching device, but through the supply or the load.
However, the resonant circuits used to provide conventional soft-switching generate large over-voltages and over-currents. The switching devices are subjected to these over-voltages and over-currents and, therefore, must be rated to withstand such stresses, a particular problem in high power applications.